1. Field of the Invention
The invention relates to a method for the continuous manufacture of chip boards, fiber boards, plastic boards and plywood and to a continuously operating press for performing the method.
2. Description of the Related Art
It is known from a number of methods and apparatus concerning the manufacture of boards to vary the compression angle in the entry gap with adjusting apparatus in the ram and/or in the press table in order to adapt it to requirements regarding the practical strength of the wood material boards and of the chip mat structure (the material to be pressed). According to German Patents 23 43 427, 31 33 792 and 34 13 396 it is known to adapt the entry gap by varying the compression angle in accordance with the practical requirements of the board to be made from wood material. Varying the pressure characteristic in the entry system, however, is possible only within narrow limits by means of a steady compression over a relatively long entry section.
Present-day practical specifications for chip boards (e.g., for varnishable surfaces) require, on the other hand, raw density profiles with a density of about 1100 kg/m.sup.2 in the top layer area of the board surface. This requires a rapid compression of the mat or raw material in the press entry gap, i.e., a steep pressure rise at the start of the pressing. According to embodiments described in German Patent 19 38 280 and in German OS 37 34 180, in the case of fiber boards, especially thin boards of 2.5 mm to 5 mm thickness, an extremely rapid top layer compression can be achieved by pre-curved convex compression sections in the entry area by controlling the pivotal supporting structure.
The development of continuous press technology with rolling support now permits the construction of very long presses with accordingly high steel belt velocities and low specific compression factors. The fast compression of the covering layers in a pre-curved entry gap, practiced according to German Patent 19 38 280, DE-OS 37 34 180 and European Patent Application 0 144 163, lead due to the higher entry velocities to blowouts along the entry width on the covering layer, due to the progressive compression in the convex entry gap, when contact with the mat begins between the upper and lower steel band. In other words, when the compression of the mat begins, the air accumulated in the extremely pre-compressed mat has to escape abruptly. Depending on the fiber structure or covering layer structure, for example in the case of chip boards with a relatively high dust content, the air permeability of the precompressed covering layer changes. If the air permeability is poor and the entry velocities are high, the air in the mat escapes explosively, forming clouds of dust or fiber which leads to streaking on the finished board at the exit of the continuously operating press. Sometimes entire areas of the covering layer, amounting for example to 100 cm.sup.2 and more, burst out of the middle layer, so that not only is the surface blemished, but also the optimum practical requirements of the finished board, e.g., laminability or flexural strength, cannot be achieved.
Precompression sections of the above-described kind, with a rigid transition in accordance with DE-OS 37 34 180 and EP-OS 0 144 163 in a straight precompression area, permit only the adaptation of the vertical compression rate to a varying permeability of the covering layers by modulating the entry and production velocities. Variations of the air escape characteristics of the covering layer cannot be compensated for without varying the rate of production; for example, if the dust content is higher or the fiber structure has poor permeability in the covering layers the production rate has to be lowered, resulting in a substantial limitation economically.
Another important limitation to be mentioned is that, if the horizontal-to-vertical compression ratio is the same at the point of contact with the chip mat, the angle of the entry arc angle with respect to the secant angle is twice as great, i.e., in comparison to a linear mat compression along a secant the rate of the escape of air at the arc due to the progressive compression along the arc is twice as great at the arc, and this intensifies the blow-out effect. The negative blow-out effect, which is disclosed in the state of the art cited above, according to the progressive compression of the mat in the precurved front entry section with a transition to the rectilinear, variable entry gap, has been minimized by the two-joint system according to DE-OS 39 18 757. However, what has not been solved is the problem of the rapid covering layer compression in thin fiber boards, because the initial compression section ahead of the first joint is substantially longer than the compression section that follows. Another disadvantage of the extremely long precompression section is that in the finished product it results in greater pre-hardening and thus more costly sanding as well as poorer raw density profiles (strength) in the covering layer.
The adjustable entry systems known heretofore have, in addition to the deficiencies described thus far, still more serious disadvantages tied to the apparatus:
a) After the chip or fiber mat makes contact with the idler drums, an extreme drop in the pressing force occurs and thus an unacceptable pre-hardening and embrittlement with cracking at the surface of the upper covering layer correspondingly occurs.
b) The same problem occurs in the apparatus according to German Patent 38 16 511. After the deflection of the steel band on the curved section of the roller frame, the steel bands are supported on friction surfaces with virtually no pressure up to the point of entry into the roller rods.
c) In the embodiment of DE-OS 37 34 180, express reference is made to an entry section separate from the main press section. This system-related apparatus is connected to the main press only by the steel bands. At the transition point, the intense application of the pressing force curve has to occur, without any controllable influence on the rest of the pressing procedure.